Machinery powered by electric motors or internal combustion engines (typically referred to as "prime movers") often use gears, shafts and the like to form a drive train providing power used to perform an end-use function. Automobiles, metal cutting and shaping machines, toggle-type presses and construction and earth-moving machines are but a few examples of such machinery.
As more specific examples, such machinery drive trains uses gears and shafts in speed reducers and right angle drives to reduce speed (from that of the motor or engine output shaft) and increase torque and/or to change the direction of power flow. Shafts interconnect "stages" of gearing or connect a final gearing stage to an output device such as automobile wheels, press head or, in the case of an earth-moving machine known as a walking dragline, to a "walk leg" drive.
In such machines, the drive train components (gears, shafts and the like) may range in size from a few pounds to several thousand pounds. Clearly, small drive train components can be more readily serviced. Equally clear is the fact that by their nature, very large drive train components (gears, shafts and the like) are usually attached together by correspondingly large devices which are very difficult to work with, at least because of their weight and size. Nowhere is the difficulty of facile repair more apparent than in large mobile machines such as earth-moving and earth-excavating machinery.
Such machinery is available in a wide variety of types ranging from the familiar rubber-tire mounted and crawler-mounted to the less-common dragline. A dragline is often used for removing top soil and "overburden" to expose a valuable mineral, e.g., coal, beneath but near the earth's surface.
Draglines are equipped with an angularly-extending boom from which is suspended a "bucket" having an open mouth and digging teeth, both toward the main portion of the machine. Overburden is removed by placing the bucket on the ground at a point distant from the machine and pulling it toward the machine, filling the bucket in the process. Once filled, the machine pivots about a central axis and the bucket emptied at a spoil pile somewhat away from the area being excavated.
Smaller draglines are crawler mounted much like a military tank and capable of movement in the same way albeit at much slower speeds. However, as draglines (and their digging buckets) increased in size, crawler PG,4 mounting was found to be impractical and in the early 1900's, the "walking" dragline was developed. The walking dragline is so named because it takes short "steps" and uses a "walk leg" mechanism (which resembles a human leg) to do so. A difference is that in a walking dragline, both legs step simultaneously.
To give some perspective to the following discussion, a large walking dragline--made by Harnischfeger Industries of Milwaukee, Wis., and incorporating the invention--has a main housing portion (including the machinery deck, operator's cab and the like) which is about 105 feet long, about 80 feet wide,, about 40 feet high and weighs about nine million pounds. The boom extends about 300 feet and the capacity of the digging bucket is about 80 cubic yards. The walk legs of such dragline take steps about seven feet in length.
And certain types of machines including walking draglines tend to load (and wear) certain of their drive train components unevenly. For example, the drive train of a toggle press (especially that portion driving the toggle press head mechanism) is most heavily loaded over only that fraction of a revolution relating to the final, piece-forming part of the press stroke. In a walking dragline, the walk leg drive is most heavily loaded only during that part of a revolution during which the dragline is being lifted to take a "step." In the exemplary dragline, gear wear occurs over only about 120.degree. or less of the total 360.degree. gear circumference. While wear over a limited portion of a gear may be one reason to use a segmented gear, it is not the only reason. The gear may be largely inaccessible, necessitating that only the "tooth parts" be removed and then only in segments.
The exemplary dragline takes advantage of a known type of gear configuration called a segmented gear. Such configuration includes a web-like, generally circular "spider" with plural arc-like gear segments mounted around the circumferential perimeter of the spider. Each segment is individually replaceable and the spider and segments may be constructed in "tongue-and-groove" fashion whereby a portion of each segment fits radially inward into a slot-like space in the spider.
The importance of securing each segment to the spider is well recognized. Earlier securing arrangements involved forming holes about the rim of the spider and along the segment and inserting bolts through such holes after they are brought into registry.
And it is important that there be no "play" in the segment-spider fit, especially radially and circumferentially, since most load stresses are radial and/or circumferential. Therefore, such earlier securing arrangements use bolts which were press-fitted into the holes. In general, the term "press-fit" means that the bolts and holes are cooperatively sized so that the bolts must be urged into and through such holes by force.
This arrangement results in several problems. When the bolts are installed, special pressing equipment is often required. And when they need to be removed for segment service (as they will, sooner or later), such bolts must be "jacked" or physically forced out of the holes over the entire bolt length, not just a portion thereof. If the bolts are of any significant size, this may require use of a hydraulic jacking tool--and a lot of patience and stamina. So-called "full-length jacking" can be partly avoided but only by fabricating bolts which are necked down in the middle. Such bolts are invariably "special," expensive to make, not as strong as bolts of uniform diameter and unlikely to be quickly available near a remote repair site.
Another problem is that in larger segmented gears, the bolts are very heavy and difficult to handle. These and other problems presented by earlier securing arrangements, and the deftness with which such problems are addressed and resolved, will be further appreciated after understanding the invention.